The sequence of reactions catalyzed by sterol 27-hydroxylase (CYP27) in the oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid was studied with apparently homogeneous preparations of the cytochrome P-450 from rabbit liver mitochondria. Conditions are described for the formation and characterization of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestane-27-al as an enzymatically generated intermediate in the oxidation process. Incubation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol or 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol with sterol 27-hydroxylase in 18O2 atmosphere resulted in the incorporation of one or two 18O atoms in the carboxyl group of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid. Similar incubations with 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestane-27-al resulted in the incorporation of one 18O atom in the 27-carboxyl group. The results strongly indicate that the sterol 27-hydroxylase performs multiple monooxygenations in the conversion of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid. The following reaction sequence (Reaction 1) at carbon 27 is proposed. [formula: see text] Reaction 1.